The present invention relates to an electrically operated heating element with a flow channel for molten plastic and a central hot rod that is connected to an outer jacket at the downstream end of the heating element, said end being provided with outflow apertures.
An electrically operated heating element of this type is known from U.S. Pat. No. 4,285,660. The hot rod is connected at its point to the outer jacket by means of webs that demarcate one or more exit apertures for the molten plastic coming from the annular flow channel. There is a section between the point and the shaft of the hot rod that has an extensive area wetted by the molten plastic emerging in the vicinity of the exit apertures. The maximum cross-section of the section is equal to or greater than that of the shaft of the rod.
The flow channel for the molten plastic in this heating element is demarcated by the central hot rod and by the outer jacket of the heating element. The cross-section of the heating element is composed of the cross-section of the hot rod, of the annular flow channel, and of the outer jacket.
An electrically operated heating element that is immersed in the hot runner that conveys the molten plastic in a hot-runner tool is also known from U.S. Pat. No. 3,520,026. That heating element has a central metal core that is connected at the end remote from the electric connection with a metal outer jacket. A layer of insulation is positioned outside of the point of connection and between the metal core and the outer jacket.
The metal core is made out of copper or a similar material and has a low effective electric resistance, whereas the outer jacket is made out of a metal with a high effective electric resistance. Thus there is a high partial voltage at the outer jacket. Since the metal core has a large cross-section, the total cross-section of the heating element is correspondingly large.